Satellite images from NASA show a surprising effect of the novel COVID-19 outbreak in China: less air pollution. Using pollution monitoring satellites, NASA measured the air’s concentration of nitrogen dioxide (NO2), a gas that ends up in the air from burning fuel, primarily from cars, trucks, buses and power plants.

From Jan. 1 to 20, and Feb. 10 to 25, researchers saw a “significant decrease” in pollution over Wuhan and the rest of China due at least in part to an “economic slowdown” resulting from the virus outbreak, according to a NASA release.

The maps above show the density of NO2 concentrations in blue, yellow and orange across China during those time periods. The reduction started in Wuhan, and then spread to the rest of the country, according to NASA. “This is the first time I have seen such a dramatic drop-off over such a wide area for a specific event,” Fei Liu, an air quality researcher at NASA’s Goddard Space Flight Center, said in a NASA release.

Typically, there are lower levels of NO2 in China around this time of year, as many businesses and factories close for the country’s Lunar New Year celebrations. The maps below illustrate how the NO2 concentrations (in blue, yellow and orange) dropped during the 2019 Lunar New Year festival, and then rose afterwards when business resumed. NASA satellite images show nitrogen dioxide concentrations in China before the Lunar New Year, during New Year celebrations and after.

However, in 2020 the NO2 levels from January to February were 10% to 30% lower than average, and they didn’t rise back in late February. Experts attribute the difference to quarantine measures that have been implemented to stop the spread of COVID-19. Millions of people in China have been on lockdown since late January, and many factories in the country closed due to the outbreak.

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In Wuhan, the capital of Hubei province where the virus originated, people have to follow strict guidelines for leaving their homes and are subject to temperature checks to go shopping for groceries. In early February, Beijing officials announced that all residents who were returning to Beijing would have to quarantine themselves for 14 days. Travel bans around the world were put in place to prevent the further spread of COVID-19.

The Centers for Disease Control issued a “level 3” travel warning for China, Iran, Italy and South Korea, meaning people should avoid all nonessential travel to these countries. “The impact of unprecedented quarantines and restrictions on travel have brought the Chinese economy right now to a virtual standstill,” Stephen Roach, a senior fellow at Yale University, told CNBC’s “Squawk Box” on Friday.

Indeed, there has been a decrease in coal consumption and transportation traffic, he added.

In the United States, there are a total of 69 confirmed cases as of Monday, and the majority of people were evacuated from Wuhan or passengers on the Diamond Princess cruise that was quarantined due to COVID-19 concerns in Japan. Globally, more than 87,000 people have been infected, and 80,000 of the cases are in China, according to the World Health Organization. The WHO hasn’t declared the COVID-19 a pandemic yet. Director general Tedros Adhanom Ghebreyesus said Feb. 24 that it “absolutely” has potential to become one.

In the U.S., people should be prepared to follow “social distancing measures,” that include working from home and school cancellations, Nancy Messonnier, head of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention, said Tuesday. Above all, the best way to prevent the spread of the virus is to wash your hands frequently and thoroughly, cover your mouth if you sneeze or cough, avoid touching your face and stay home if you’re sick, according to the CDC.